Variable stiffness devices and methods of use

ABSTRACT

Variable stiffness devices and methods of their use are provided. In some embodiments, a variable stiffness device comprises an inner member defining a compartment for receiving an actuating fluid; an outer member disposed around the inner member; and a granular medium disposed between the inner member and the outer member; wherein the inner member is being moveable in a radial direction from a relaxed state to an expanded state by introducing the actuating fluid into the compartment of the inner member to compress the granular medium against the outer member to increase the stiffness of the device.

RELATED APPLICATIONS

This application is a continuation patent application of U.S. patentapplication Ser. No. 15/154,443, filed May 13, 2016, which claims thebenefit of and priority to U.S. Provisional Patent Application No.62/161,865, filed on May 14, 2015, each of which are incorporated hereinby reference in their entireties.

FIELD

The present disclosure relates to variable stiffness devices withvariable stiffness and shape, and methods of their use.

BACKGROUND

There are many potential uses for devices with variable stiffness andshape, such as active suspensions in automobiles, wearable devices, andcivil engineering structures. For example, in stroke patients, loss ofmuscle function is common and causes significant weakness in the lowerleg. In severe cases there may be complete paralysis of the legs.Therefore, in case of post-stroke and injury rehabilitation, it may berequired that the load on such type of weak bones and muscles is kept toa minimum. Variable stiffness devices can be used to provide motionassistance to stroke patients, with the stiffness being varied asnecessary to strengthen their muscles and bones. Accordingly, there is aneed for devices that can provide a range of stiffness, while being easyto use.

SUMMARY

Fluid operated devices with variable stiffness and shape and methods ofuse are provided. A device may comprise one or more inner members madefrom an elastic material and defining a compartment for receiving anactuating fluid, the inner members being moveable in a radial,longitudinal direction or both from a relaxed state to an expanded stateby introducing an actuating fluid into the inner members; and an outermember made from an inelastic material and being disposed around theinner members to control expansion of the inner member in a radial orlongitudinal direction, and a granular material disposed between theouter and inner members, which is compressed by the inner membersagainst the outer member to increase the stiffness of the device.

In some embodiments, a variable stiffness device comprises an innermember defining a compartment for receiving an actuating fluid; an outermember disposed around the inner member; and a granular medium disposedbetween the inner member and the outer member; wherein the inner memberis being moveable in a radial direction from a relaxed state to anexpanded state by introducing the actuating fluid into the compartmentof the inner member to compress the granular medium against the outermember to increase the stiffness of the device.

In some embodiments, the inner member is made from an elastic materialand the outer member is made from a non-stretchable material. In someembodiments, the outer member has a fixed size in the radial directionand a variable size in a longitudinal direction. In some embodiments,the outer member is reinforced on a side to direct bending of the outermember under a load. In some embodiments, the outer member is pre-bentto a side to direct bending of the outer member under a load. In someembodiments, the granular medium is compressible. In some embodiments,the granular medium is a mixture of rubber pellet granules ranging fromabout 1 to about 3 mm in diameter. In some embodiments, the granularmedium has a packing factor between about 0.55 and about 0.74 atatmospheric pressure. In some embodiments, the variable stiffness deviceis incorporated into an article of clothing.

In some embodiments, a variable stiffness device comprises a first innermembers, the first inner member defining a first compartment forreceiving a first actuating fluid; a second inner member wrapped aroundthe first inner member, the second inner member defining a secondcompartment for receiving a second actuating fluid; an outer memberdisposed around the inner member; and a granular medium disposed betweenthe second inner member and the outer member; wherein the first innermember is configured to expand in a longitudinal direction byintroducing the first actuating fluid into the first compartment of thefirst inner member to expand the variable stiffness device in thelongitudinal direction; and wherein the second inner member isconfigured to expand in a radial direction by introducing the secondactuating fluid into the second compartment of the second inner memberto compress the granular medium against the outer member to increase thestiffness of the variable stiffness device.

In some embodiments, a method for providing structural support to astructure comprises disposing a variable stiffness device in connectionwith a structure in need of structural support, the variable stiffnessdevice comprising an inner member defining a compartment for receivingan actuating fluid; an outer member disposed around the inner member;and a granular medium disposed between the inner member and the outermember; expanding the inner member by introducing the actuating fluidinto the compartment of the inner member to compress the granular mediumagainst the outer member; and adjusting the amount of the actuatingfluid in the compartment to vary an amount of structural supportprovided by the variable stiffness device.

DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained withreference to the attached drawings. The drawings shown are notnecessarily to scale, with emphasis instead generally being placed uponillustrating the principles of the presently disclosed embodiments.

FIG. 1 illustrates a example of an actuator system and variablestiffness device according to the present disclosure;

FIG. 2A and FIG. 2B illustrate a side view of one embodiment of avariable stiffness device of the present disclosure in a relaxed state(FIG. 2A) and stiffened state (FIG. 2B).

FIG. 2C and FIG. 2D illustrate a cross-sectional view of one embodimentof a variable stiffness device of the present disclosure in a relaxedstate (FIG. 2C) and stiffened state (FIG. 2D).

FIGS. 3A-3B illustrate an embodiment of a rotary joint employingvariable stiffness devices of the present disclosure.

FIGS. 4A-4C illustrate another embodiment of a variable stiffness devicehaving multiple inner members.

FIG. 5 illustrates another embodiment of a variable stiffness device ofthe present disclosure.

FIG. 6 illustrates an embodiment of an article of clothing employingvariable stiffness devices of the present disclosure.

FIG. 7 is a force as a function of displacement graph comparing pointsof failure (structure starts to bend) of variable stiffness devices atvarying fill pressures.

FIGS. 8-9 present modeling data for a controller configuration tocontrol a variable stiffness device of the present disclosure.

While the above-identified drawings set forth presently disclosedembodiments, other embodiments are also contemplated, as noted in thediscussion. This disclosure presents illustrative embodiments by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of the presently disclosedembodiments.

DETAILED DESCRIPTION

The present disclosure provides a variable stiffness device that hasvariable stiffness and can be used to mimic the overall performance ofbone. According to some aspects of the present disclosure, there isprovided a fluid operated (hydraulic and pneumatic) device comprising aninner member made from an elastic material and defining a compartmentfor receiving an actuating fluid, an outer member made from aninelastic, non-stretchable material and being disposed around theelastic inner member, and a granular medium disposed between the innermember and the outer member. The devices of the present disclosure maybe pressurized either with liquid (hydraulically) or with gas(pneumatically). The outer member may have a fixed maximum length so itcannot be extended or stretched beyond such maximum length. The innermember can be moveable in a radial direction, longitudinal direction, orboth from a relaxed state to an expanded state by introducing theactuating fluid into the compartment of the inner member to compress thegranular medium against the outer member to increase the stiffness ofthe device.

In some embodiments, the present devices may include multiple innermembers to control the length of the device and the stiffness of thedevice. For example, a fluid operated device of the present embodimentmay comprise a first inner member made from an elastic material anddefining a compartment for receiving an actuating fluid, a second innermember made from an elastic material and defining a compartment forreceiving an actuating fluid, an outer member made from an inelasticmaterial, being disposed around the first inner member and the secondinner member, and a granular medium disposed between the outer memberand the first inner member and second inner member. In some embodiments,the second inner member may be wrapped around the first inner member. Insome embodiments, the second inner member may be spiral in shape orcylindrical shell shaped. The first inner member may be moveable in thelongitudinal direction from a relaxed state to an expanded state byintroducing the actuating fluid into the compartment of the first innermember to increase the length of the device. The second inner member maybe moveable in a radial direction from a relaxed state to an expandedstate by introducing the actuating fluid into the compartment of thesecond spiral inner member to compress the granular medium against theouter member to increase the stiffness of the device.

FIG. 1 illustrates a non-limiting embodiment of a hydraulic system 100for use with the present variable stiffness devices 200. In someembodiments, the hydraulic system 100 may include a pump 110, one ormore valves 120, a controller 130 and one or more variable stiffnessdevices 200 that can act as an artificial bone with variable stiffness.The actuator system 100 may further include a reservoir 150 for a fluidused to actuate the elastic actuator. In operation, the pump may be usedto pump the actuating fluid from the reservoir into the variablestiffness devices 200 to change the stiffness of the variable stiffnessdevices 200, as is described in more detail below. Once a desiredpressure is achieved, the variable stiffness devices 200 may bemaintained at the desired pressure and stiffness to support a load. Whenthe fluid is discharged from the elastic actuator, the pressure releasefrom the actuator causes the variable stiffness devices 200 to decreasein stiffness and become more flexible. The controller 130 is incommunication with the component of the actuator system 100, as shown inFIG. 1, to control the operation of the actuator system 100. Thecontroller can be any type of controller known and used in the art. Itshould be noted that other hydraulic systems can also be used. In someembodiments, a pneumatic system may be employed to operate the variablestiffness devices of the present disclosure. For example instead of apump, reservoir, and valves, in some embodiments, direct fluid volumemanagement may be used, such as for example, utilizing motorized leadscrew in series with syringe's plunger, or a hydraulic single acting ordouble acting cylinder. In some embodiments, a simple user powered pumpcan be also utilized for manual pressurization of the present devices.

In reference to FIGS. 2A-2D, in some embodiments, the variable stiffnessdevice 200 includes an inner member 220 surrounded by an outer member210 with a layer of granular medium 230 disposed between the innermember 220 and the outer member 210. In some embodiments, the innermember 220 forms an elongated, expandable compartment for receiving anactuating fluid. The inner member 220 can thus be moved from a relaxedstate, as shown in FIG. 2A and FIG. 2C, to an expanded or pressurizedstate, as shown in FIG. 2B and FIG. 2D, by introducing the actuatingfluid into the inner member 220 and back to the relaxed state upondischarge of the actuating fluid from the inner member 220. Pressurizingthe inner member 220 may expand the inner member 220 in the radialdirection, longitudinal direction or both. As the inner member expands,it may compress the granular medium against the outer member to changethe stiffness of the variable stiffness device 200. When the innermember 220 is not pressurized or pressurized at small fluid pressure,the inter granular distance is large enough that granules can easilypass next to each other such that the structure is easily bendable andcharacterized with small stiffness. On the other hand, at higherpressures, the inter granular distance is small, i.e. granular media isjammed such that structure is rigid and characterized with largestiffness.

The inner member 220 may be made of expandable materials, preferablyhaving one or more of the following characteristics: resistance to wearand tear, high tensile strength, resilience, and elongation. The outermember 210 may form an outer sleeve around the inner member 220 todirect or control the expansion and contraction of the inner member 220.The outer member 210 may be made of a variety of different materials,preferably having one or more of the following characteristics: clothlike inelastic, tough, with low absorption of moisture and a highflexural strength, in radial direction, linear direction or both. Insome embodiments, the outer member may be made of polyester. In someembodiments, the outer member 210 may be rigid or non-stretchable in theradial or outward direction but compressible in the axial orlongitudinal direction. In some embodiments, the outer member may becorrugated or may utilize a telescoping mechanism, for example, withconcentric cylinders. In some embodiments, the outer member 210 has afixed size in the radial direction and a variable size in a longitudinaldirection. In some embodiments, the outer member 210 may contract bywrinkling or bending under compressive load.

The variable stiffness devices 200 may linearly or axially contractunder compressive load and exhibit variable stiffness behavior, whichdirectly correlates to amount of fluid pressure applied within the innermember 220. During contraction the outer member may wrinkle. Ifcompressive load is too large for given pressure the device 200 maybuckle to one side. In some embodiments, one side of variable stiffnessdevice outer member 210 can be reinforced, in part or in whole. In someembodiments, the reinforcement may be used to direct bending in adesired direction. In some embodiments, the variable stiffness devices200 of the present disclosure can be slightly pre-bent even withoutcompressive load such that variable stiffness device naturally bends,i.e. buckles, to a desired side. In some embodiments, one side of thevariable stiffness device outer member 210 can be reinforced with rigidelement to prevent buckling.

The buckling for typical support element applications is consideredundesirable. However, for applications where variable stiffness deviceis considered part of the rotary joint the controlled buckling may beconsidered desirable. In some embodiments, the variable stiffnessdevices 200 may be utilized as part of a rotary joint, and one or moreof variable stiffness devices 200 with and without buckling may beutilized to generate variable stiffness rotary dynamics undercompressive load. FIGS. 3A illustrates an embodiment of a rotary jointwith multiple variable stiffness devices 200, without buckling. FIG. 3Billustrates an embodiment of a rotary joint with a variable stiffnessdevice with a single variable stiffness device 200, with a reinforcement209 on one side to induce controlled buckling or bending to a desiredside.

In some embodiments, the inner member 220 is comprised of an elasticmaterial allowing for expansion of the inner member 220 in the radialdirection. In some embodiments, the inner member 220 may be formed fromlatex material, and the outer member may be made from polyester ornylon. The outer member 210 is comprised of an inelastic material whichis configured to limit expansion of the inner member 220 in the radialdirection. The outer member 210 is made of an inelastic material toallow the compression of the granular medium 230 against it and to limitthe diameter of the device. When the actuator fluid is delivered to theinner member 220, the inner member expands and the granular medium 230are compacted and compressed between the inner member 220 and the outermember 210, in a manner known as granular jamming. This allows thegranular medium 230 to move past each other and leave a flexible device200 when unpressurized, but creating a much stiffer device 200 with ahigher compressive strength when the device 200 is pressurized.

The outer member 210 may be designed to prevent or at least minimizeradial expansion of the inner member 220. To that end, it is desirablethat when the outer member 210 is expanded there are no openings in theouter member 210 through which the granular medium 230 or any portion ofthe inner member 220 can protrude in radial direction when pressurized.In some embodiments, the outer member 210 can be made from a sheet ofmaterial which in the expanded state has no openings to provide anunbroken or uninterrupted barrier which prevents the granular medium 230or the inner member 220 from protruding through the outer member 210, inwhole or in part. By way of a non-limiting example, the outer member 210may be made from a corrugated fabric or cloth like material. Theexpansion of such outer member is not likely to create any openings inthe outer member through which the granular medium 230 or thepressurized inner member 220 may protrude, in whole or in part. In someembodiments, the outer member 210 is allowed to expand in longitudinaldirection, if necessary, by unfolding the folds of the outer member 210,rather than by simply stretching the outer member 210, which may resultin unwanted openings.

In some embodiments, the granular medium 230 may be compressible, suchas rubber, such that the device 200 will get stiffer as the pressure ofthe actuator fluid delivered to the inner member 220 is increased. Insome embodiments, the granular medium may include granulated EPDM(ethylene propylene diene monomer) rubber for the granular media due tothe inexpensive cost, ease of sourcing, and lack of reactions withwater. In some embodiments, other granular medium can be used dependingon the desired strength or stiffness of the variable stiffness devices,type of materials, average particle size, particle geometry, and thepacking factor of the granular medium.

In some embodiments, elastic, ethylene propylene diene monomer (M-class)rubber (EPDM) granule may be utilized. In some embodiments, rigidsilicon sand granules can be used. In some embodiments, the elasticgranular medium can be a mixture of rubber pellet granules ranging fromabout 1 to about 3 mm in diameter. For granules not being perfectlyspherical in shape, the granular medium can exhibited similar propertiesin terms of packing factor ranging in between 0.55 and 0.74 inunpressurized state (i.e. at atmospheric pressure) depending on appliedpacking as observed for granular medium with spheres. Uponpressurization the packing factors can reach close to 0.95 packingfactor due to compressible nature of rubber particles.

In reference to FIGS. 4A-4C, in some embodiments, the device 200 maycomprise multiple inner members to control the length and stiffness ofthe device. FIG. 4A illustrates a cross section of the device in arelaxed state when neither inner member is filled with actuating fluid.FIG. 4B illustrates a cross section of the device in a lengthened statewhen the first inner member 220 is filled with actuating fluid to changethe length of the device. FIG. 4C illustrates a cross section of thedevice in a lengthened and a stiffened state when the first inner memberand the second inner member 221 are filled with actuating fluid. Forexample, the device may include an outer member 210 and a first innermember 220, a second inner member 221, and granular medium 230 placed ina layer between the outer member 210 and both the first inner member 220and second inner member 221.

The first inner member 220 may control longitudinal, i.e. axial, i.e.linear length and the second inner member 221 may provide variablestiffness. The granular medium 230 may be selected such particles arenot allowed to pass each other when the second inner member 221 ispressurized. The first inner member 220 may be comprised of an elasticmaterial and be primarily configured to allow for expansion of thedevice in the longitudinal direction. The second inner member 221 mayalso comprise of an elastic material but be primarily configured toallow for expansion in the radial direction. In some embodiments, thesecond inner member 221 is spiral in shape and wrapped around the firstinner member 220. In some embodiments, the second inner member 221 isdisposed coaxially with the first inner member 220. The outer member 210is comprised of an inelastic, non-stretchable material which isconfigured to limit expansion of the device 200 in the radial direction.

When the actuator fluid is delivered to the first inner member 220, asdemonstrated in FIG. 4B, the first inner member may expand the device200 in the longitudinal direction. When a desired length is achieved,the second inner member 221 may be pressurized, as demonstrated in FIG.4C, to expand the second inner member 221 in the radial direction. Thismay cause the granular medium 230 to become compacted and compressedbetween the first inner member 220, the second inner member 221, and theouter member 210, which changes the stiffness of the device. This allowsthe granular medium 230 to move past each other and leave (they don'tleave, they are just moving easily past each other!) a flexible device200 (maybe have “and device 200 is soft and bendable” instead of “leavea flexible device 200”) when unpressurized, but creating a much stifferdevice 200 with a higher compressive strength when the device 200 ispressurized. In some embodiments, the inner members may be concentricwith each other.

In reference to FIG. 5, one or more variable stiffness devices of thepresent disclosure may be combined. In some embodiments, a variablestiffness device 200 includes two independent variable stiffnessdevices, comprising an outer member 210 a, 210 b and an inner member 220a, 220 b and granular medium 230 a, 230 b disposed between the outermember and the inner member. In some embodiments, the multiple variablestiffness devices are disclosed concentrically with one another, asshown in FIG. 5, but they can also be disposed in differentconfiguration.

In some embodiments, the variable stiffness devices of the presentdisclosure may be used to provide structural support to a structure. Thevariable stiffness devices of the present disclosure may be disposed inconnection with such structure and adjusted to a desired stiffness toprovide a level of structural support required or desired for thestructure. In some embodiments, the present variable stiffness devicesmay be incorporated into the structure in need of support. In someembodiments, the present variable stiffness devices may be disposedaround the structure or otherwise in connection with the structure. Insome embodiments, the devices of the present disclosure may beincorporated into articles of wearable clothing, braces, prostheticsockets or fittings, walkers, crutches or exoskeletons, so the devicescan help support one or more joints or other body parts of the user.

The devices of the present disclosure can be produced very inexpensivelyand can have a variety of applications from wearable medical devices tostand-alone robotics systems. The devices of the present disclosure canbe used in a different industries and technologies, such as, forexample, the health industry, medical device technologies for humans andanimals, aerospace and automotive technologies, space technologies,underwater technologies, robotic system technologies and similarindustries and technologies, as well as for use in exoskeletons,prosthetics and orthotics, furniture, construction, underwater, aero,space and civil engineering. In some embodiments, the devices of thepresent disclosure may be incorporated into an article of robotic systemwherein the device is configured to help support one or more joints orother body parts of a robot. In some embodiments, the devices of thepresent disclosure may be incorporated into an article of activesuspension and impact management for automotive system, wherein thedevice is configured to help support and manage forces onto one or moreelements of automotive system. In some embodiments, the devices of thepresent disclosure may be incorporated into an article of structuralsupport for furniture (chairs, beds etc) and tent elements, wherein thepresent device can be configured to help support and manage forcesapplied onto these structure and/or user.

In reference to FIG. 6A, in some embodiments, one or more variablestiffness devices 200 of the present disclosure may be combined with awearable article of clothing 610, to form an exoskeleton. When the userwears the article of clothing 610, the one or more variable stiffnessdevices 200 can be positioned substantially along the clothing to bestprovide support to the user in the manner of representing an artificialbone. For example, the devices may help support or protect user's jointswhen carrying a load. In some embodiments, the device may be shaped toconform to the shape of the user's limbs for additional comfort. In someembodiments, multiple devices may be incorporated into a wearableclothing 610 to help protect multiple joints of the user. In someembodiments, the device when combined with a wearable article ofclothing 610 is configured to have a light support structure, i.e.variable shape and stiffness, utilizing the stiff, variable stiffnessdevice, which can be tuned “on the fly” to transition from being a verystiff, rigid support to a completely soft, bendable material. The sizeand shape can also be also tuned during operation. The exoskeletondevice may be adjustable for specific users and/or intended tasks ofspecific users.

It should be noted that the devices 200 of the present disclosure mayalso be used to assist in muscular movement of the wearer. For example,as discussed above, the inner member may be expandable in thelongitudinal direction, with or without expansion in the radialdirection by introducing the actuating fluid therein and may be allowedto contract in upon discharge of the actuating fluid. In this manner,the contracting movement of the inner member can be used as the muscleforce, creating a tensile force similarly to an actual biological muscleto, for example bend the limbs of the joint. Additionally oralternatively, the variable stiffness devices of the present disclosuremay be combined in the exoskeleton with variable stiffness devices thatmimic muscle movement, such as for example, disclosed in U.S.application Ser. No. 14/628,663, which is incorporated herein byreference in its entirety.

In some embodiments, the variable stiffness devices may be controlledusing an automatic controller. The experimentally establishedrelationship between fluid pressure and critical buckling force, i.e.minimal compressive force that could cause device to buckle, can beutilized for automatic control of stiffening or softening of the presentvariable stiffness devices based on applied compressive force. Theautomatic control can be used to prevent buckling of the variablestiffness devices under compressive load and also to soften device asmuch as necessary. For applied compressive forces that are larger thandemarcation force, defined as, for example, 25N below critical bucklingforce for given fluid pressure, the system can be configured to increasethe fluid pressure and hence increase the critical buckling force suchthat force difference in between critical force and measured force islarger than 25N. Similarly if the measured compressive force issubstantially smaller than demarcation force, the system can beconfigured to lower fluid pressure, hence lowering the buckling force,and therefore lowering the demarcation force. Other controlmethodologies may be used to control stiffness by varying pressure inrelation to critical buckling force as well as in relation to otherdesired metrics (e.g. user comfort).

Examples, which are set forth to aid in the understanding of thedisclosure, and should not be construed to limit in any way the scope ofthe disclosure as defined in the claims which follow thereafter. Thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the embodiments of the present disclosure, and are notintended to limit the scope of what the inventors regard as theirinvention nor are they intended to represent that the experiments beloware all or the only experiments performed. Efforts have been made toensure accuracy with respect to numbers used (e.g. amounts, temperature,etc.) but some experimental errors and deviations should be accountedfor.

EXAMPLES Example 1

Devices were constructed using granulated EPDM (ethylene propylene dienemonomer) rubber for the granular media. The potential for use of othergranulation is possible, with the likelihood that the device's strengthwould change according to certain properties, such as the material used,average particle size, particle geometry, and the packing factor.Previous tests have utilized coffee, salt, and hydrophobic sand asdifferent granulations.

A test device was built, filling the excess space with the rubberpellets before being sealed at both end with a pipe clamp. The rubbertubing was connected to ½ in. NPT threaded connectors, which were thenscrewed into machined aluminum block end-pieces. The tubing used hadinner and outer diameters of ½ and ⅞ inches, respectively. The finalouter diameter was approximately 1.5 inches.

A secondary hole was placed in the base of one block to serve as theinput for water to pressurize the device, and was sealed off duringtesting. Testing was completed at the three following pressures, 0 (nowater), 70, and 110 PSI. Pressures were confirmed with a gauge prior totesting, but were not rechecked during or after testing was complete.

For the testing, an Instron 5567A machine running the Blue Hill 2Software with flat compressive grips was used to place a load on thesample. The crosshead speed compressed at a rate of 0.2 in/min, and wasmanually stopped after it was felt that the critical data was collected,usually around one inch of compression. Collected data included time,position, and force in pounds. The results can be seen in FIG. 7.

Points circled in black on FIG. 7 are considered approximate points offailure, as the testing setup was not capable of identifying a singlepoint in time that served as failure. Failure was classified as bucklingof the device, with there being bending in the center, and a noticeablyreduced increase in the compressive force for an equal amount ofdisplacement. Peak forces for the three curves were about 5, 17, and 35lb for the pressures of 0, 70, and 110 PSI, respectively.

Testing results show that device stiffness increases in correlation withan increase in the internal fluid pressure. Additionally, the datasuggests the higher stiffness correlates with a greater displacementneeded to reach failure. The test completed at 0 PSI (no water in thedevice) can be considered to have failed from the start, as the devicequickly buckled during compression, and offered very little resistiveforce, as can be seen in FIG. 4. However, for example the experimentallyobtained Young's Modulus of same device increased from 0.70 MPa at 0.21MPa fluid pressures, to 3.53 MPa at 0.55 MPa fluid pressures, which isfive-fold increase in stiffness over moderate range of fluid pressure.In comparison, pressurizing the device up to 110 PSI can significantlyincrease the strength of the device.

Example 2

The operation of a control system were modeled. The system included aninlet pump in series with the variable stiffness device and a branchedexit controlled by a solenoid valve. A controller was designed andimplemented to control the solenoid valve, based on the critical bendingforces collected on the variable stiffness device. An Arduino Unomicrocontroller was used in order to actuate the solenoid valve. Thecritical bending forces for the variable stiffness device at differentpressures were plotted and a 2nd order polynomial curve fit was appliedto compute the equation of critical bending force as a function ofpressure (Pressure (MPa) Vs. Critical Load (N)), as shown in FIG. 8.

In the real time implementation of the controller, the pressure and loaddata were collected and the critical bending force for the correspondingpressure was calculated using the following equation:

F _(C)=715.5P ²+155.7P+10.44   (1)

An allowed load threshold was provided such that the actual forceexerted on the bone was 9.071 kg (20 lb) lower than the critical bendingat all times. This ensured that the variable stiffness device was underthe linear loading regime to prevent the system from buckling andsubsequently cause a reduction in the critical bending force. The errorbetween the load acting on the variable stiffness device and the allowedload was calculated and minimized for control. If the external loadacting on the system changes, the new allowed load is recalculated andthe exit valve is opened or closed respectively. As shown in FIG. 9, themeasured load, allowed bucking and the critical buckling force overtime.

To test the controller performance, a load of 9.071 kg (20 lb) wasrandomly applied to and removed from the load cell [18] and the systemresponse time was determined. From FIG. 11, it can be inferred that theaverage response time to pressurize the bone was 3.37 seconds while theaverage time during depressurization was 1.19 seconds.

The entirety of the tests performed on the variable stiffness devicerevealed that the measured load was lower than the allowed bucklingforce of the system. With the system set up having a virtual loadthreshold of 9.071 kg added to it, the critical buckling force of thesystem was never reached.

TABLE 1 Mean, Standard Deviation, Minimum, and Maximum values for Errorvolumetric flow rates at no load. Volumetric Error Flow Rate Std.(cm³/sec) Mean Deviation Min Max 12.2 0.455 0.736 −0.562 1.687 15 0.2970.589 −0.621 1.288 24.6 0.838 0.784 −0.522 2.109 44.4 0.937 0.796 −0.5352.413 92.3 0.876 0.797 −1.461 2.254

The controllability of the system was measured under various flow rates.Due to the fixed opening of the solenoid valve, the exit flow ratesvaried significantly even though valve was opened instantaneously.Therefore, individual no-load tests for the error were performed atvarying flow rates and the mean, standard deviation, minimum, andmaximum errors are listed in Table 1.

All patents, patent applications, and published references cited hereinare hereby incorporated by reference in their entirety. It should beemphasized that the above-described embodiments of the presentdisclosure are merely possible examples of implementations, merely setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiment(s) without departing substantially from the spirit andprinciples of the disclosure. It will be appreciated that several of theabove-disclosed and other features and functions, or alternativesthereof, may be desirably combined into many other different systems orapplications. All such modifications and variations are intended to beincluded herein within the scope of this disclosure, as fall within thescope of the appended claims.

What is claimed is:
 1. A wearable device comprising: an article ofclothing and one or more variable stiffness devices associated with thearticle of clothing, wherein at least one of the one or more variablestiffness devices comprises: an inner member moveable in a radialdirection from a relaxed state to an expanded state; an outer memberdisposed around the inner member, the outer member having a fixed sizein the radial direction; and a granular medium disposed between theinner member and the outer member; wherein, as the inner member movesfrom the relaxed state to the expanded state, the inner membercompresses the granular medium against the outer member to increase thestiffness of the variable stiffens device.
 2. The wearable device ofclaim 1 wherein the inner member defines a compartment for receiving anactuating fluid, and is moveable from the relaxed state to the expandedstate by introducing the actuating fluid into the compartment of theinner member.
 3. The wearable device of claim 1 wherein the outer memberhas a variable size in a longitudinal direction.
 4. The wearable deviceof claim 1 wherein the inner member is made from an elastic material andthe outer member is made from a non-stretchable material.
 5. Thewearable device of claim 1 wherein the outer member is reinforced on aside to direct bending of the outer member under a load.
 6. The wearabledevice of claim 1 wherein the outer member is pre-bent to a side todirect bending of the outer member under a load.
 7. The wearable deviceof claim 1 wherein the granular medium is compressible.
 8. The wearabledevice of claim 1 wherein the granular medium is a mixture of rubberpellet granules ranging from about 1 to about 3 mm in diameter.
 9. Thewearable device of claim 1 wherein the granular medium has a packingfactor between about 0.55 and about 0.74 at atmospheric pressure.
 10. Awearable device comprising: an article of clothing and one or morevariable stiffness devices associated with the article of clothing,wherein at least one of the one or more variable stiffness devicescomprises: a first inner members, the first inner member defining afirst compartment for receiving a first actuating fluid; a second innermember wrapped around the first inner member, the second inner memberdefining a second compartment for receiving a second actuating fluid; anouter member disposed around the inner member; and a granular mediumdisposed between the second inner member and the outer member; whereinthe first inner member is configured to expand in a longitudinaldirection by introducing the first actuating fluid into the firstcompartment of the first inner member to expand the variable stiffnessdevice in the longitudinal direction; and wherein the second innermember is configured to expand in a radial direction by introducing thesecond actuating fluid into the second compartment of the second innermember to compress the granular medium against the outer member toincrease the stiffness of the variable stiffness device.
 11. Thewearable device of claim 10 wherein the outer member has a fixed size inthe radial direction and a variable size in a longitudinal direction.12. The wearable device of claim 10 wherein the outer member isreinforced on a side to direct bending of the outer member under a load.13. A method for providing structural support to a structure, the methodcomprising: disposing a variable stiffness device in connection with astructure in need of structural support, the variable stiffness devicecomprising an inner member defining a compartment for receiving anactuating fluid; an outer member disposed around the inner member; and agranular medium disposed between the inner member and the outer member;expanding the inner member by introducing the actuating fluid into thecompartment of the inner member to compress the granular medium againstthe outer member; and adjusting the amount of the actuating fluid in thecompartment to vary an amount of structural support provided by thevariable stiffness device.
 14. The method of claim 13 wherein the innermember is made from an elastic material and the outer member is madefrom a non-stretchable material.
 15. The method of claim 13 wherein theouter member has a fixed size in the radial direction.
 16. The method ofclaim 15 wherein the outer member has a variable size in a longitudinaldirection.
 17. The method of claim 13 wherein the outer member isreinforced on a side to direct bending of the outer member under a load.18. The method of claim 13 wherein the outer member is pre-bent to aside to direct bending of the outer member under a load.
 19. The methodof claim 13 wherein the granular medium is a mixture of rubber pelletgranules ranging from about 1 to about 3 mm in diameter.
 20. The methodof claim 13 wherein the granular medium has a packing factor betweenabout 0.55 and about 0.74 at atmospheric pressure.
 21. The method ofclaim 13 further comprising incorporating the variable stiffness deviceinto an article of clothing.